The term “operating wavelength” of an optical element refers to the wavelength at which the reflectance of the multilayer system is at its maximum.
Optical elements comprising a substrate and a multilayer system which is optimised for high reflectance at a particular wavelength, e.g. photomasks or mirrors for the extreme ultraviolet (EUV) wavelength range, e.g. wavelengths between 11 nm and 16 nm, are in particular required for use in EUV lithography of semiconductor devices.
Such optical elements can also be used as beam control elements or monochromatic illuminators right up into the x-ray range. In the most simple case, a multilayer system comprising a stack of double layers made of a highly absorbent and a poorly absorbent material (with different complex refractive indices), simulates an ideal crystal which meets the Bragg condition, wherein the absorbent layers correspond to the reflection planes in the crystal, while the less absorbent layers function as spacers. For a given material, the thickness of the individual layers is constant across the thickness of the multilayer system. Thus, the thickness ratio in a double layer is also constant. Depending on the requirements of the mirror in relation to the reflection profile, all types of other multilayer systems are however also possible. For example, bandwidth and reflectance can be set in that the multilayer system is made from more than just two materials (e.g. triple layers, quadruple layers, etc.), or that the layer thickness or the thickness ratio of the layers in said multilayer system is not constant (so-called depth-graded multilayers).
In the EUV spectral range, the predominant systems are molybdenum/silicon or molybdenum/beryllium systems. For a standard description of multilayer systems we refer for example to Eberhard Spiller, Soft X-Ray Optics, SPIE Optical Engineering Press, Bellingham, Wash. 98227, USA, ISBN 0-8194-1655-X, ISBN 0-8184-1654-1 (soft). EP 1 065 532 A2 provides an example of highly reflective mirrors for the EUV range, wherein, in order to improve reflectance, the periodicity is broken through by additional layers made of other materials.
As a first layer of the multilayer system, optical elements can comprise special layers on the substrate, with the function of said special layers consisting predominantly of providing the best possible surface finish for the subsequent layers (so-called buffer layer), since in particular in the EUV range, small areas of roughness or ripples can bring about deviations from the ideal layer thickness, such deviations having a noticeable impact on reflectance.
Typical EUV lithography equipment comprises eight or more mirrors. In order to nevertheless achieve adequate overall transmission to the incident radiation, the reflectance of the mirrors must be as high as possible, because the overall intensity is proportional to the product of the reflectance of individual mirrors. If at all possible, the mirrors should maintain this high reflectance during their entire service life. This is an important factor in reducing production costs of e.g. semiconductor components and optical components. With photomasks, too, contamination is a big problem, because the structures to be imaged are no longer in focus as a result of deposits e.g. of carbon. In this document, contamination refers to deposits on the originally free interface, e.g. carbon, and chemical reaction below the free interface, e.g. oxidation. Furthermore, there are irradiation and heat effects which negatively affect the reflectance at “operating wavelength”.
Water deposits have their origin in the water fraction contained in the residual gas. Carbon deposits have their origin in hydrocarbons which are outgassed from individual device components or from the photoresist with which the wafer to be exposed to light is coated. For example, irradiation of EUV radiation causes an accumulation of oxide or elemental carbon, because after the reaction under EUV irradiation neither the oxygen (from the oxide) nor the (elemental) carbon, can return to the vacuum.
The correlation between carbon contamination of mirror surfaces and the emission of photoelectrons has already been investigated in H. Boiler et al., Nuclear Instruments and Methods 208 (1983) 273–279. It was found that hydrocarbons which accumulate on the mirror surface are broken down by secondary electrons generated near the free interface, and that as a result of this, the deposition of a permanent carbon layer on the mirror surface is facilitated. Although Boller does not mention this, by analogy, oxidation can be expected in residual gas with a high water content. According to Boller, the secondary electrons are essentially generated independently of the irradiated wavelength, the structure of the irradiated surface, and the angle of incidence.
J. H. Underwood et al., SPIE, vol 3331, pp. 52–61, show that photoemission, via the frequency-dependent reflection peak of multilayer systems experiences a minimum and a maximum, and that the intensity of the photoemission is proportional to the intensity of the electrical field of the standing wave which forms when the incident irradiation is reflected. From these experiments, it is concluded that a phase shift of π/2 occurs during reflection, between the maximum and the minimum of the field intensity and thus the photoelectric current. Photoemission reacts sensitively to the contamination. Underwood et al. propose that the chemical and physical characteristics of the surface layers be examined by measuring the photoelectrons. This article does neither discuss the influence that the multilayer system has on the photocurrent characteristics, nor its influence on the contamination speed.
S. Oesterreich et al. SPIE, vol. 4146, pp. 64–71, present an investigation of the reflectance of multilayer mirrors, in particular molybdenum/silicon mirrors during EUV irradiation. They note that matching the wavelength of the incident irradiation to the layer thickness (provided the uppermost layer has not oxidized and is of ideal thickness) leads to minimal field intensity at the mirror surface. This is reported to result in the contamination layer probably growing more slowly, thus having less influence on the reflectivity. On the other hand, matching of the wavelength leads to operation outside the operating wavelength, which in turn leads to loss of reflectance. This study does not discuss the relationship between photoemission/electrical field intensity on the one hand, and photoemission/contamination on the other hand.
EP 1 065 568 A2 proposes an EUV lithography apparatus comprising an illumination system for illuminating a photomask, a photomask holding device, a projection system for imaging the photomask onto a substrate, and a substrate holding device, wherein the illumination system and/or the projection system comprise(s) at least one multilayer mirror which comprises a protective layer made of inert material(s).
Preferred materials are diamond-like carbon, boron nitride, boron carbide, silicon nitride, silicon carbide, boron, palladium, ruthenium rhodium, gold, magnesium fluoride, lithium fluoride, tetrafluoroethylene, as well as their compounds or alloys. These protective layers are particularly resistant to oxidation and are characterised by low absorption in the EUV range. The preferred thickness of the layers is between 0.5 nm and 3 nm. A protective layer can comprise individual sub-layers (e.g. two) so as to achieve the best possible reflectance. However, any other contamination of the mirror, apart from oxidation, is only suppressed insignificantly or is not suppressed at all.
The situation is similar in the protective passivation layers for mirrors used in EUV lithography, disclosed in WO 99/24851. A protective layer comprises at least two sub-layers, with an outer layer made of palladium, carbon, carbide, boride, nitride or an oxide; and a subjacent layer made of silicon or beryllium. The thickness of the sub-layers is optimised with a view to good reflectance.
U.S. Pat. No. 6,231,930 discloses protective layers against oxidation, for optical surfaces in lithographic applications, in particular in the EUV range. The surface to be protected is introduced to an atmosphere comprising a gas which contains carbon with functional groups with good surface absorption. During irradiation, the gas molecules are broken down into particles which form a cohesive layer, 0.5 nm to 2 nm in thickness, wherein said cohesive layer adheres well to the surface by way of the functional groups, while being unsuitable as an absorption base for the molecules and molecule fractions that remain in the residual gas of the vacuum. Thus, growth of the protective layer ceases automatically.
U.S. Pat. No. 5,307,395 describes an attempt to increase the service life of a multilayer mirror for the soft X-ray range, in that the layer thickness, in particular in the uppermost thickness, is optimized so that the standing wave which forms, is positioned in a targeted way in relation to the layer boundaries. In this arrangement, a node of the standing wave is positioned exactly in the middle of the uppermost layer. This results in the absolute maximum of the field intensity, and thus the maximum of the absorption, being shifted to the interior of the mirror. As a consequence, damage such as high tension or cracks resulting from excessive irradiation load (approx. 2 to 4*108 W/cm2) is prevented. In the case of EUV mirrors, in particular in lithography equipment, such damage occurs extremely rarely because irradiation load is less severe by several magnitudes (approx. 1 W/cm2).